Diesel engines efficiently convert the latent heat of hydrocarbon fuel into useful mechanical power. In operation of conventional diesel engines, a metered amount of fuel is injected into each cylinder of the engine at recurrent intervals synchronized with rotation of the engine crankshaft to coincide with the air-compression stroke of a reciprocating piston. As pressure increases, the compression temperature in the cylinder rises and the injected fuel is soon hot enough to ignite. The resulting combustion or firing of fuel in the cylinder forces the piston to move in the opposite direction, thereby applying torque to the engine crankshaft.
Conventional engine fuel is a relatively low grade, refined petroleum known generally as diesel fuel oil which has desirable ignition and heat release characteristics. Diesel fuel oil has acceptably low levels of corrosive, abrasive and other noxious matter, and it is in ample supply at the present time.
For nearly a century persons skilled in this art have known that coal, in the form of either a dry powder or a liquid slurry (i.e., a mixture of pulverized coal or other form of carbon dust and a liquid carrier such as oil or water), is an alternative fuel for diesel engines. Interest in developing a practical coal-fueled diesel engine has varied over the years directly with the cost and inversely with the supply of standard diesel fuel oil. For a review of such development efforts, see the article entitled "Slow-Speed Two-Stroke Diesel Engine Tests Using Coal-Based Fuels" by J. P. Davis, J. B. Dunlay, M. K. Eberle, and H. A. Steiger, published in 1981 as paper No. 81-DGP-12 by the American Society of Mechanical Engineers (New York, N.Y., U.S.A.).
The injection of a coal-water slurry (hereinafter sometimes referred to as "CWS") into a compression ignition reciprocating internal combustion engine such as a large, medium-speed, multi cylinder diesel engine, poses problems not typically encountered in the injection of pure liquid fuels. One problem is that CWS does not ignite as readily as conventional diesel fuel because it has a relatively long ignition delay time, because there are practical limits to the degree of atomization of CWS that can be obtained, and because there are practical limits in the amount that the inlet air temperature and the compression temperature of the engine cylinders can be increased compared to diesel engines using standard diesel fuel oil as their primary fuel.
More than 65 years ago it was recognized that a small amount of readily ignitable pilot fuel could be injected in diesel engines to improve combustion of "heavy" hydrocarbon fuels that are otherwise difficult to ignite. See British Patent No. 124,642. As used herein, the term "pilot fuel" means relatively light hydrocarbon fuel (e.g. methanol or even standard diesel fuel oil) characterized by being significantly easier to ignite than the primary fuel in the injection system.
U.S. Pat. No. 4,825,842 disclosed a fuel injection system for an internal combustion engine in which diesel oil was injected as an ignition fuel and in which CWS was injected as a primary fuel. That patent discloses an apparatus which permits the use of diesel fuel either as a pilot fuel for CWS, or as the sole fuel for the engine. It does not, however, address the issue of the timing of injection of the primary CWS fuel and the ignition fuel, and it does not address the optimization of fuel efficiency, emission control and maximum cylinder pressure using those fuels.
U.S. Pat. No. 4,612,898 discloses a cylinder head for a piston internal combustion engine having two fuel injection nozzles, one of which is used to inject an ignition fuel and the other of which is used to inject a main, non-self-igniting fuel. It does not, however, address the issue of the timing of injection of the primary CWS fuel and the ignition fuel, and it does not address the optimization of fuel efficiency, emission control and maximum cylinder pressure using those fuels.
U.S. Pat. No. 4,700,672 discloses a two fuel injector apparatus for an internal combustion engine which may be used for a main fuel which is a liquid or gaseous fuel and a pilot fuel. It teaches that the pilot fuel may be injected simultaneously with the main fuel. It does not, however, address the issue of the timing of injection of a primary CWS fuel and an ignition fuel, and it does not address the optimization of fuel efficiency, emission control and maximum cylinder pressure using those fuels.
U.S. Pat. No. 4,782,794 to Hsu et al disclosed that a small amount of readily ignitable pilot fuel could be injected prior to the injection of CWS to aid the combustion of the CWS fuel in a coal-fueled diesel engine. It suggested that the pilot fuel could be introduced by mixing it with the CWS in the fuel supply tank; or a separate pilot fuel injector could be used (U.S. Pat. No. 4,335,684); or the pilot and main injectors could be combined in one coaxial assembly (see U.S. Pat. No. 4,266,727). It also suggested that fuel costs would be saved (assuming that CWS fuel is less expensive than pilot fuel) by injecting the smallest amount of pilot fuel consistent with timely ignition of the CWS fuel.
U.S. Pat. No. 4,782,794 to Hsu also discloses a fuel injection system particularly adapted for injecting coal slurry fuels at high pressures includes an accumulator-type fuel injector which utilizes high-pressure pilot fuel as a purging fluid to prevent hard particles in the fuel from impeding the opening and closing movement of a needle valve, and as a hydraulic medium to hold the needle valve in its closed position. A fluid passage in the injector delivers an appropriately small amount of the ignition-aiding pilot fuel to an appropriate region of a chamber in the injector's nozzle so that at the beginning of each injection interval the first stratum of fuel to be discharged consists essentially of pilot fuel and thereafter mostly slurry fuel is injected.
Several articles that are of some general interest in the general subject matter of diesel engines or in the use of CWS fuel in combustion engines are listed below:
Annand, W. J. D., "Heat Transfer in the Cylinder of Reciprocating Internal Combustion Engines," Proc. Instn. Mech. Engrs., Vol. 177, No. 36, 1963.
Caton, J. A., Kihm, K. D., Seshadri, A. K. and Zicterman, G., "Micronized Coal Water Slurry Sprays from a Diesel Engine Positive Displacement Fuel Injection System," Presented to the Combustion Institute, Central States Section, 1991 Spring Technical Meeting, Nashville, Tenn., April, 1991 (hereinafter "[Caton, 1991]").
Flynn, P. L., Hsu, B. D., and Leonard, G. L., "Coal Fueled Diesel Engine Progress at GE Transportation Systems," ASME Publication, Journal of Engineering for Gas Turbines and Power, Vol. 112, No. 3, 1990, pp. 369-375 (hereinafter "[Flynn et al, 1990]").
Hsu, B. D., "Heat Release, Cycle Efficiency and Maximum Cylinder Pressure in Diesel Engine--The Use of an Extended Air Cycle Analysis," S. A. E. Transactions, 1984, p. 4.766 (herinafter "[Hsu 1984]").
Hsu, B. D., "Progress on the Investigation of Coal-Water Slurry Fuel in a Medium Speed Diesel Engine: Part 1--Ignition Studies," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 110, No. 3, 1988, pp. 415-422 (hereinafter "[Hsu 1988a]").
Hsu, B. D., "Progress on the Investigation of Coal-Water Slurry Fuel in a Medium Speed Diesel Engine: Part 2--Preliminary Full Load Test," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 110, No. 3, 1988, pp. 423-430 (hereinafter "[Hsu 1988b]").
Hsu, B. D., Leonard, G. L., and Johnson, R. N., "Progress on the Investigation of Coal-Water Slurry Fuel in a Medium Speed Diesel Engine: Part 3--Accumulator Injector Performance," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 111, No. 3, 1989, pp. 516-520 (herinafter "[Hsu et al 1989]").
Hsu, B. D. and Confer, G. L., "Progress on the Investigation of Coal-Water Slurry Fuel Combustion in a Medium Speed Diesel Engine: Part 4--Fuels Effect," ASME Publication, Coal Fueled Diesel Engines, ICE Vol. 14, 1991 (herinafter "[Hsu 1991]").
Kanury, A. M., Introduction to Combustion Phenomena, Gordon and Breach Science Publishers, second edition, 1977 (hereinafter "[Kanury, 1975]").
Wahiduzzaman, S., Blumberg, P. N. and Hsu, B. D., "Simulation of Significant Design and Operating Characteristics of a Coal Fueled Locomotive Diesel Engine," ASME Publication, Coal Fueled Diesel Engines, ICE Vol. 14, 1991 (herinafter "[Wahiduzzaman 1991]").
Walsh, P. M., Zhang, M., Farmayan, W. F., Beer, J. M., "Ignition and Combustion of Coal-Water Slurry in a Confined Turbulent Diffusion Flame," presented at the 20th International Symposium on Combustion, Ann Arbor, Mich., Aug. 1984.
The following methods of igniting CWS in a diesel engine were discussed in Hsu, B. D., "Progress on the Investigation of Coal-Water Slurry Fuel in a Medium Speed Diesel Engine: Part 1--Ignition Studies," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 110, No. 3, 1988, pp. 415-422. (hereinafter "[Hsu 1988a]"):
Compression ignition, in which CWS is ignited solely by the compression temperature generated inside the engine cylinder; PA1 Separate pilot diesel fuel injection, in which a separate pilot injector was used to supply a small amount of pure diesel fuel to ignite the CWS which was injected through the main injector (using two separate injectors); and, PA1 Stratified pilot fuel ignition, in which a small amount of diesel fuel is delivered to the cylinder through the main fuel injector where the first part of the fuel discharged from the injector consisted essentially of diesel fuel, followed by mostly CWS. Where pilot fuel was used, it was injected prior to the injection of the CWS fuel to aid ignition of the CWS fuel.
The Hsu 1988a article described ignition studies of CWS fuel in a medium speed diesel engine in which the CWS fuel and pilot fuel were separately injected into the combustion chamber using separate injection systems. All of the tests were conducted under low load conditions. In the tests described in that article, the pilot fuel was injected either before the CWS fuel or at or near the beginning of the injection of the CWS fuel under low load conditions.
In tests reported in [Hsu 1988a] and in [Flynn et al., 1990], preliminary success was obtained with a converted mechanical fuel injection equipment (FIE) 12 cylinder engine burning mostly coal slurry fuel. However, the mechanical fuel injection equipment used by that engine could provide only about 95% combustion efficiency, and had to use high percentage of diesel pilot fuel. Hsu, B. D., "Progress on the Investigation of Coal-Water Slurry Fuel in a Medium Speed Diesel Engine: Part 2--Preliminary Full Load Test," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 110, No. 3, 1988, pp. 423-430. Flynn, P. L., Hsu, B. D., and Leonard, G. L., "Coal Fueled Diesel Engine Progress at GE Transportation Systems," ASME Publication, Journal of Engineering for Gas Turbines and Power, Vol. 112, No. 3, 1990, pp. 369-375 (hereinafter "[Flynn et al., 1990]").
In a previous paper published by Hsu [1988a], it was pointed out that when pilot fuel is used to ignite CWS fuel under low or minimum load ignition conditions, the starting time of combustion of the CWS fuel is dictated by the start of pilot fuel ignition.
As reported earlier in Hsu, B. D., Leonard, G. L., and Johnson, R. N., "Progress on the Investigation of Coal-Water Slurry Fuel in a Medium Speed Diesel Engine: Part 3--Accumulator Injector Performance," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 111, No. 3, 1989, pp. 516-520 (hereinafter "[Hsu 1989]"), a high pressure electronically controlled accumulator injector using a diamond compact insert nozzle [Flynn et al., 1990] was developed. The improved reliability and durability of this new FIE allowed for an improved and more thorough study of combustion of CWS fuel in a diesel engine. It was decided to include a diesel pilot fuel injector in the combustion system mainly due to engine start and low load operation needs. As a result, the experimental combustion study was very much facilitated due to the ability of changing pilot/CWS injection timings and quantities without having to stop the engine. Other parameters studied included combustion chamber configuration (by changing CWS fuel injector nozzle hole number/shape/angle), as well as injection pressure.
As the result of extensive testing, it has been determined that relatively small amounts of diesel fuel may be introduced into the combustion chamber prior to CWS fuel at minimum load conditions as a pilot fuel to ignite CWS fuel. Under mid-range to maximum load conditions, however, injection of the same amount of diesel fuel in the conventional way as a pilot fuel before the injection of CWS fuel does not make the CWS fuel burn in a timely and clean manner. Although it might be possible to introduce a large amount of diesel fuel to achieve timely and clean combustion of the CWS fuel, that would partially defeat the purpose of using the less expensive CWS fuel to run the engine.